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Windows 7: Question about USB 2.0 and 3.0 cables

I have several USB 2.0 and USB 3.0 portables wihich came with USB 2.0 and USB 3.0 cables respectively.
One of my recent computer tower has both USB 2.0 and USB 3.0 ports.
I understand that USB 3.0 and 2.0 cables are compatible with either USB 2 or USB 3 ports.

My question is about the speed of file transfer and other functions when USB 2.0 cable (with portable drive with USB 2.0 port) is plugged into USB 3.0 ports in the tower , and vice versa (USB3.0 cable to USB 2.0 port).

Any device will run no faster than it is designed to be and no faster than the port it is plugged into. In your example, the USB 3.0 device will run no faster than the USB 2.0 port does.

Something to keep in mind about USB speed standards that they are a range. For example, USB 3.0 is basically any speed that is faster than USB 2.0 up to 5.0 Gbps. An inexpensive USB 3.0 device may not run much faster than a high priced USB 2.0 devices.

Also keep in mind that not all USB ports will run at maximum speed for the standard nor be able to provide the maximum amount of power for the standard.

The USB 3.0 cables that came with my external drive enclosures don't look like any USB 2 cable I've ever seen. Not the end that plugs into the device anyway. The other end that plugs into the PC looks the same but has extra pins in it.

Keep in mind that even though a device advertises as being USB 3.0 compatible, that doesn't mean it will run at full USB 3.0 speed. My USB 3.0 drive enclosures have 5400 RPM laptop drives in them. They won't even reach full USB 2.0 speeds. I'd have to put an SSD in them to even come close to USB 3.0 speeds.

Plus there is another problem. The USB3 controllers seem to 'eat' a lot of the 5Mbps. In a series of tests I made, the eSata port was faster than the USB3 port using the same devices - the eSata port is rated at only 3Mbps.

The USB port, cable, end device, plus any intermediate USB devices such as a hub, effectively form a chain. And as is well known the strength of a chain is determined by that of the weakest link. The same principle applies to many aspects of computer performance. If any of the links in the chain are designed only for USB 2, that is the weakest link and will determine transfer speed. The performance characteristics of the stronger links in the chain don't matter. If any device is substandard in quality or operating under adverse conditions it may further reduce performance.

To obtain full USB 3 performance everything in the chain must be of good quality and designed for USB 3.

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